Low ohmic contacts for power HEMTs (high electron mobility transistors) and other types of heterostructure devices are needed to meet a low RON*A (specific on-resistance, where A corresponds to area) metric. This is especially true for low voltage power devices (30V voltage class and below), where the contact resistance can represent 40% or more of the total device RON*A. Preferably, a low ohmic contact in an AlGaN/GaN HEMT or other heterostructure device has low contact resistance (and therefore low impact on RON) and also low transfer length. If the transfer length is low, the length of the contact can be reduced significantly and the size of the power transistor structure can be correspondingly reduced. However, it is very difficult to provide good ohmic contacts at a heterojunction such as a GaN/AlGaN interface. For example, an optimized 30V GaN power transistor typically has a specific contact resistance of 1.2e-7 Ohm*mm2 which corresponds to about 40% of the total transistor RON*A. Additionally, special care must be taken to optimize the transfer resistance between the 2DEG (two-dimensional electron gas) channel and the contact. This transfer resistance has a major impact on the overall contact resistance.
One type of conventional GaN/AlGaN HEMT contact is formed by implanting Si into the GaN/AlGaN structure to form a degenerated region in contact with the 2DEG channel (Si acts as an n-type dopant in GaN). A metal contact is formed on the top side of the semiconductor body in contact with the Si doped region. Enough electrical carriers are provided below the metal contact to obtain a good ohmic contact. However, this contact structure has a high transition resistance at the underlying GaN/AlGaN interface which significantly increases the overall specific resistance of the contact area. The high transition resistance arises due to a well pronounced barrier between the GaN/AlGaN interface caused by band discontinuity and induced/spontaneous polarization charges.
Another type of conventional GaN/AlGaN HEMT contact is formed by metal deposition and subsequent annealing performed at typically high temperatures above 600° C. Such high temperature processing prohibits the use of standard aluminum metallization schemes which have melting points below 600° C. With GaN based materials, such high temperature annealing creates nitrogen vacancies under the buried metal contact. These nitrogen vacancies act like n-type dopants in GaN, creating a similar effect as with a conventional Si implanted contact. A recess etch can be performed down to or even below the 2DEG channel to avoid the transition resistance at the GaN/AlGaN interface. However, the buried metal contact structure is in direct contact with the 2DEG channel. Such a direct connection between a metal contact and a 2DEG channel causes current crowding at the channel-metal interface and increases the specific contact resistance.